The property industry, desperate to keep the boom inflated for as long as possible, is doing all in its power to stop the minister in his tracks. Its opening salvo, delivered last week by IIB Bank chief economist Austin Hughes, rubbished the idea that greedy speculators are driving up prices by snapping up properties off the plans and selling them out at a quick profit before the builders have even moved on site.

To find the real villains, Hughes says you need look no further than the population bulge, a wave of immigrants and easy credit such as 100% and 40-year mortgages. These fundamentals, rather than crafty speculators, are behind the continuing surge in house prices according to Hughes, warning that ham-fisted meddling by government in the property market would be a disaster.

Yet there is no denying that a lot of people chasing property have no plans to live in the houses they are buying.

Some are buy-to-let investors, although their numbers are dwindling as rents fail to keep pace with soaring property prices and mortgage payments that are being pushed upwards as interest rates rise.

Some are the owners of holiday homes that may eventually be used as retirement bolt holes.

But others are simply making hay while the boom lasts, making a tidy killing by sitting on properties that will never be occupied by tenants or by the owners themselves.

This last category might not fit easily in to Ahern's description of wheeler-dealer speculators, flipping off-plan properties every few weeks. But they are an important if largely overlooked force in the market, whose actions could have serious knock-on consequences when property prices eventually come off the boil.

Hughes says the growth in vacant dwellings is nothing unusual, adding that it it simply another area where Ireland is playing catchup with other countries that have been richer for longer. While acknowledging that vast swathes of empty housing may be socially damaging, especially in seaside locations that close down after the summer holidays, he says they do not pose an economic danger. "I don't think there's an overhang of vacant property that's going to come on the market all at once, " he says.

The big problem in second-guessing what might happen is that nobody can be sure exactly how many empty houses there are, the reasons they are unoccupied, and what their owners might react if the property market ran out of steam.

"According to the 2002 census, 11.7% of the housing stock was vacant. That's 170,000 houses, " says Dermot O'Leary, economist at Goodbody Stockbrokers. "Now it seems to have gone above 16% according to preliminary data from this year's census. That's a pretty big jump in five years."

Nevertheless, we still have some way to go to catch up with our European neighbours.

One-in-three houses lie empty in Spain and Greece while in Italy and France the proportion is one-in-five. This reflects the huge number of holiday homes along the Mediterranean and, when you travel north to countries less dependent on tourism, you find far fewer vacant homes. For example, only 6.2% of the British housing stock is empty while less than 5% of German houses have no lights burning at night.

Back at home, O'Leary says the gap between house building and demand from local owner-occupiers is especially wide in parts of the west and midlands. For example, one-in-five houses were empty on census night in 2002 along the Atlantic seaboard from Donegal to Kerry.

"This indicates to us that a pretty large proportion of construction is for holiday home purposes, " he says. "It's also driven by the various tax schemes and rural renewal relief has led to rapid growth in house completions in Leitrim and Longford which is not accounted for by household formation in these areas."

O'Leary does not foresee problems - at least not yet. "Vacant housing is a bit of a black hole in the overall property statistics but it isn't something we need to be wary of right now, " he says. "Demand on the ground doesn't indicate there's an overhang of property anywhere in the market. But it is something to watch out for, especially towards the end of the decade as housing supply catches up with demand."

The trouble is that, because information on the housing market is so sketchy, we don't know how worried we should be. "I do think there's an issue with vacant properties but we don't know enough to know how serious it could be, " says Pat McArdle, chief economist at Ulster Bank. "You'd have be concerned about vacant investment property especially if it was bought with tax breaks.

People do stupid things to avoid paying tax and often they never set foot in the properties they are buying."

In a country that has grown rich quickly from property, it is impossible to predict what will happen when the party finally ends.

While a slowdown in house prices would not involve widespread financial hardship, it would dent the confidence that has led many to believe that property is a guaranteed money spinner.

"If capital appreciation stopped for a year or two, a brief period of zero growth, the psychological satisfaction from owing property would no longer be there, " McArdle says. But this does not mean that people sitting on vacant houses would immediately dump them on the market.

"If you bought a year ago you're up 15%-20% already so, even if prices took a big drop, you'd only be back to square one, " says McArdle. "You'd also have to ask yourself what you'd do with the money if you sold up. A lot of people are still afraid of the stock market while overseas property is not an easy option."

John FitzGerald of the Economic and Social Research Institute has done more than anyone else to get a handle on the size of the vacant property market. He was the first to slice and dice the numbers from the 2002 census but says it will be sometime next year before he will be able to repeat the exercise for the data gathered in 2006.

Nevertheless, he believes we know enough to be reassured that vacant property is not the weakest link in the property chain. "We're unlikely to see a sudden sell off if there was a downturn, " says FitzGerald.

"The tax investors can't sell up before a certain date or they'll lose the tax breaks. And other investors will probably want to hang on to the properties for personal use, even if they remain vacant for most of the time. In a downturn, it would be the demand from new investors that would fall off rather than existing investors selling into a dieing market."

Nevertheless, FitzGerald has come up with some proposals that could seriously rattle the market. These include a tax on empty and second homes or, failing that, a charge on new holiday homes built in remote areas to reflect the true cost of hooking them up to public services such as water, electricity and roads.

While this would be politically risky, especially in the run-up to a general election, it is not completely out of the question. "People would be more willing to accept it than a tax that affected all properties, " FitzGerald says, following last month's call by UCD economist Colm McCarthy to bring back a form of residential property tax.

"It would also be politically popular in places such as Donegal, where there is a growing backlash against holiday homes. I wouldn't rule it out, especially in the medium term."